Wireless communications systems enable users of User Equipment (UE) to communicate with other such users via one of a number of base stations and a core network. Typically, the UEs are mobile terminals, such as cellular telephones and the like. In an active or connected state a UE is registered with the network and has a Radio Resource Control (RRC) connection with a base station so that the network knows to which base station (or cell thereof) the UE belongs and can transmit data to and receive data from the UE. The base station (i.e. the serving cell) provides mobility information and security parameters for the user equipment. The UE also establishes a default Evolved Packet System (EPS) Bearer at an endpoint beyond the base station, typically a gateway, in the Enhanced Packet Core (EPC) network, or core network for short. An EPS Bearer defines a transmission path through the network and assigns an IP address to the UE, at which it can be reached by other communication devices, such as another UE. An EPS Bearer also has a set of data transmission characteristics, such as quality of service, data rate and flow control parameters, which are defined by the subscription associated with the UE and are established by the Mobility Management Entity (MME) upon registration of the UE with the network.
The EPS Bearer is thus managed by the MME, which signals to the UE when it needs to activate, modify, or deactivate a particular EPS Bearer. Thus there are always two connections between the UE and the communication network: one for the user data transmitted using the established EPS bearer (also known as the user plane) and another one for managing the EPS Bearer itself (also known as the control plane).
As part of the Long Term Evolution (LTE) of UTRAN (UMTS Terrestrial Radio Access Network) referred to as E-UTRAN, there are plans to introduce a feature of proximity based (e.g. direct device-to-device (D2D) or locally routed) communication when the UE can communicate user data to another UE that is within the transmission range of (or served by the same base station as that of) the first UE without the need to use LTE core network resources. Such services can be achieved by establishing a special, relaying EPS bearer to be used by the mobile communication devices in direct or locally routed communication instead of their default or other conventional EPS bearers (which might still be used for other types of communications). This direct or locally routed communication would result in better utilization of the available resources, especially on the radio interface, where these are limited. A so-called Proximity-based Services (ProSe) functionality has been specified in the 3GPP TR 22.803 (v 12.1.0) standards document, the contents of which are incorporated herein by reference.
More recently the provision of a relay functionality, in the UE, using the ProSe functionality has been proposed to allow one UE to relay the signalling for another UE to and from the network.
Due to the added complexity represented by the above functionalities and the like, it is important to ensure that each network element operates in conformance with the applicable set of standards (in this case the 3GPP standards). Since proximity based services significantly increase the complexity of the mobile communication devices (and other user equipment), there is a strong desire to test implementation of these (and similar) functionalities by the mobile communication devices before they are released to the market (or whilst they are in use in a communications network). However, the testing of such UE specific functions needs to be supported by the mobile communication device as well.
As known to a person skilled in the art, a so-called System Simulator (SS) entity may be used for testing compliance (and/or performance) of a network element, such as a base station or user equipment. The SS entity does this by simulating the operation of another network element (i.e. that would otherwise communicate with the device to be tested in a ‘live’ network). In addition to this, the SS entity controls the overall test process, including the operating parameters of a mobile communication device (in case the mobile communication device is being tested), such as activating/deactivating features, and setting an operating mode (e.g. normal mode, test mode, etc.).
The SS entity activates the test mode for the mobile communication device in which the SS entity is able to set up (remotely) one or more communication paths terminating in a so-called loopback function of the mobile communication device (from where the data units originating from the SS entity are forwarded or ‘looped-back’ to the SS entity). This effectively means that the SS entity is able to monitor communications by the mobile communication device taking place via the looped back communication path(s). Then, by monitoring and analyzing the communication taking place using the communication path(s) terminating in the loopback function, it is possible to determine whether or not the behaviour of the device being tested is in line with what is prescribed in the relevant set of standards (in which case conformance to that set of standards can be certified and/or the product can be released to the market). However, if the behaviour of the device being tested is not in line with the relevant set of standards, it is possible to determine suitable actions (e.g. re-design/re-configuration/ re-calibration of that device) so that conformance to the relevant set of standards can be achieved before using the device in a ‘live’ communication system.
The relevant mechanisms developed by 3GPP for special conformance testing functions are defined in TS 36.509 (for user equipment in E-UTRA frequency division duplex (FDD) mode and time division duplex (TDD) mode) and in TS 34.109 (for user equipment in UMTS system, for FDD and TDD modes). The contents of both documents are incorporated herein by reference.
In summary, the above specifications describe the functions and their activation/deactivation methods that each LTE/UMTS mobile communication device is required to implement for conformance testing purposes.
The UE test loop function provides access to isolated functions of the mobile communication device via the radio interface without requiring the presence and/or usage of a physical connection (e.g. a hardware interface) between the mobile communication device and the SS entity for conformance testing. However, it will be appreciated that the mobile communication device may also be connected to the SS entity using wired communication means (e.g. a Radio Frequency (RF) cable), for example, to avoid/reduce interference during test and hence increase test reliability.
For E-UTRA, the following loop modes have been defined:                UE test loop mode A        UE test loop mode B        UE test loop mode C        
UE test loop mode A provides loopback of data packets (e.g. Packet Data Convergence Protocol (PDCP) Service Data Units (SDUs)) for bi-directional data radio bearers while the UE is operating in E-UTRA mode. The downlink PDCP SDUs received by the UE on each bi-directional data radio bearer are returned on the same radio bearer regardless of the PDCP SDU contents and the Traffic Flow Template (TFT) of the associated EPS bearer context.
UE test loop mode B provides loopback of PDCP SDUs (E-UTRA and UTRA), Sub Network Dependent Convergence Protocol (SNDCP) Protocol Data Units (PDUs) (in GSM/GPRS) and Radio Link Protocol (RLP) PDUs (CDMA2000) for bi-directional EPS bearers while the UE is operated in E-UTRA, UTRA, GSM/GPRS or CDMA2000 modes. UE test loop mode B can not be used when more than one PDN connection is established or more than one primary Packet Data Protocol (PDP) context is active. When operating in E-UTRA, UTRA or GSM/GPRS then the downlink PDCP SDUs or SNDCP PDUs received by the UE on all bi-directional data radio bearers are returned by the UE on the data radio bearer associated with an EPS bearer context with a TFT matching the Transmission Control Protocol (TCP)/User Datagram Protocol (UDP)/Internet Protocol (IP) information within the PDCP SDU or SNDCP SDU. When operating in CDMA2000 modes, the downlink RLP PDUs received by the UE on all bi-directional data radio bearers are returned by the UE on the data radio bearer with the smallest identity, regardless of the RLP PDU content and the TFT of the associated EPS bearer context.
UE test loop mode C provides counting of successfully received Multimedia Broadcast and Multicast (MBMS) Packets on a given MBMS traffic channel (MTCH) while the UE is operating in Evolved MBMS (E-MBMS)/E-UTRA mode.